Aluminum coating process



Dec. 8, 1959 ALLAN CHIN ET AL 2,916,397

ALUMINUM COATING PROCESS Filed April 22, 1958 United States Patent ALUMINUM COATING PROCESS Allan Chin and Jack E. Woolley, Pittsfield, and John D. Warren, Hinsdale, Mass, assignors to General Electric Company, a corporation of New York Application April 22, 1958, Serial No. 730,232

7 Claims. (Cl. 117-51) The present invention relates to a coating of ferrous metal articles, and more particularly to a hot-dip process for providing an aluminum coating on ferrous metal articles.

Various types of hot-dip aluminizing processes wherein a metal part is dipped into a molten aluminum bath are already well known. In such processes, the surface preparation of the ferrous article prior to the dipping step is an important factor in producing continuous, adherent aluminum coatings and in ensuring adequate corrosion resistance of the coated product. In the past, it has been necessary to hot-dip the ferrous article immediately after the surface has been cleaned, because leaving the cleaned surface exposed to the atmosphere for an appreciable period prior to dipping adversely affected the adherence of the aluminum coating subsequently applied. Moreover, care had to be taken to avoid touching the cleaned surface, since fingerprints and other types of contamination led to poorly bonded coatings on such contaminated areas. The use of surface fluxes of various types, for example chlorides and fluorides, for protecting or ensuring .clean surfaces has not proved satisfactory, due to such problems as contamination of the aluminum by the flux materials, the difficulty in handling the harsh fluxes and flux-coated articles without risk to operating personnel, and the adherence of flux particles to the coated part which marred its surface and often made it more susceptible to corrosion effects.

A further problem has been the difficulty in obtaining uniform aluminum coatings in joints or narrow spaces in ferrous parts of complex shape, even when the part was dipped immediately in the molten aluminum after being cleaned, and as a result these areas have been particularly susceptible to corrosion under operating conditions.

It is an object of the invention to provide an improved process of coating ferrous metal articles with aluminum which overcomes the above disadvantages and which is readily and economically carried out.

It is a particular object of the invention to provide an improved pre-treatment of ferrous metal articles prior to aluminizing in a hot-dip process which permits a considerable period to elapse between the cleaning and dipping steps without sacrifice in the quality or adherence of the coating produced and without the use of corrosive or contaminating fluxes such as heretofore employed.

It is another specific object of the invention to provide an improved pre-treatment of ferrous metal articles having joints or other relatively complex areas to be aluminized in a hot-dip process, for facilitating the application of the aluminum coating material in the crevices, corners and other difficultly accessible regions of the ferrous article.

With the above objects in view, the present invention relates to a process of coating a ferrous metal body with aluminum which comprises thoroughly cleaning the surface of the ferrous metal body, applying on the cleaned surface a coating comprising an alkylolamine, such as monoethanolamine, and drying, and thereafter immersing 2,916,397 Patented Dec. .8, 1959 lowing description taken in conjunction with the accompanying drawing, in which the single figure shows a metal article fabricated of joined parts, wherein the coating method of the present invention may be advantageously employed.

The figure illustrates in cross section a portion of a ferrous structure presenting comparatively complex surface areas and joints which have been successfully aluminized by the present process. The structure comprises a transformer tank wall 1 to which an elongated cooling fin 2 is joined, such as by seam welding, along the region 3, the article as a result of the hot-dip procedure being completely coated on its exposedsurfaces with a thin aluminum layer 4. Of particular significance is the formation of continuous void-free aluminum coatingsS in the crevices between the welded members 1 and 2.

The procedure herein described may be applied to ferrous metal parts generally, and iron and steel articles of a Wide variety of shapes, sizes and intricate configuration have been efiectively provided with continuous, corrosion-resistant aluminum coatings using the present process. Transformer tanks of distribution type, mounting brackets, lifting lugs, cooling fin assemblies, and insulator bell caps are examples of items which have been satisfactorily aluminized. 1

In preparing the ferrous piece for the hot-dipping process, it is extremely important to thoroughly clean its surface. The cleaning procedure may be in accordance with conventional practice and may comprise such steps as grit blasting of the surface to remove heavy rust and scale, removal of grease and oil, pickling in an acid solution of suitable composition and concentration, and any other cleaning procedure found necessary.

Heretofore, it had been the practice to dip the part in the molten aluminum bath as soon as possible after-the pre-cleaning step, since it was necessary to avoid exposure of the cleaned surface to the atmosphere for any appreciable period of time. This was because the readily reactive nature of the bright, cleaned surface tended quickly to form an oxide coating thereon which prevented good adherence of the aluminum coatings subsequently applied.

In accordance with the invention, the ferrous metal immediately after being cleaned is treated with a solution of an alkylolamine (hydroxy alkyl amine) so as to cover the entire surface to be aluminized with a thin coating of the alkylolamine solution, and-the coating allowed to dry. In this condition, it has been found that the thus treated ferrous metal may remain exposed to the air for at least 24 hours before dipping it into the molten aluminum bath, and that it may be handled with the bare hands without adversely affecting the adherence or continuity of the aluminum coating thereafter applied. The latitude thus afforded in the time between cleaning and hotdipping has considerably facilitated large scale aluminizing processes and reduced the number of deflectively coated parts where delay in dipping was unavoidable.

It was further found that the alkylolamine coating did not have a contaminating effect on the aluminum in the dipping bath and no degradation of the bath material by the alkylolamine was observed even after a considerable number of dippings had been made. This is in contrast to the accumulation of impurities in the aluminum bath previously experienced in the use of inorganic fluxes and protective materials heretofore commonly used for treating the ferrous articles before the dipping procedure. The improvement afforded in this respect may reside in the fact that the alkylolamine is of organic composition and readily decomposes into volatile compounds upon being subjected to the elevated temperature of the molten aluminum bath, e.g., about 1300 F. or above, and no residual compounds remain to contaminate the aluminum bath or mar the surface of the coated ferrous article.

Monoethanolamine and triethanolamine in aqueous solutions of about .l-% by volume have proved particularly satisfactory for providing the necessary protection for the cleaned surface of the ferrous articles. However, other alkylolamines, e.g., diethanolamine, may suitably be used, and the particular concentration of the alkylolamine in the solution applied does not appear critical. The range specified above affords a solution which is conveniently and economically prepared and applied, but much higher concentration and even relatively pure alkylolamine solutions which are liquid or semi-liquid at ordinary temperatures may be employed within the scope of the invention. While aqueous solutions are preferred, solvents of other types such as known organic solvents may be used if desired in preparing the alkylolamine solutions.

In a typical process carried out in accordance with the invention, a ferrous metal part of relatively complex configuration was initially subjected to steel grit blasting to remove scale and loose rust. The part was then vapor degreased in perchlorethylene to remove oil and grease, this stage also serving to preheat the part for the acid cleaning step. The degreased part was immersed in an acid pickling bath, composed of a 10% sulphuric acid solution, for about 3 minutes at 80 C., the acid bath also preferably containing a pickling inhibitor of known type to avoid excessive attack on the metal. The acidcleaned part was then rinsed with a spray of cold water. Immediately after this procedure, the cleaned part was immersed in a bath of 1% (by volume) aqueous monoethanolamine solution at room temperature for 1 to 20 minutes. The part was removed from the monoethanolamine bath and allowed to dry in the air, using an air blast or fan. With the dried monoethanolamine coating thus covering the surface, the part could be handled or left exposed to the air for a period up to about 24 hours. The part was then immersed in a bath of substantially pure aluminum for about 3 minutes at 1320 F., removed from the bath, and the excessive aluminum allowed to drain from the surface. If desired, the part could be shaken or subjected to an air blower to facilitate removal of the molten aluminum drops. In the usual case a continuous aluminum coating varying from 2 to 6 mils in thickness, with an average of about 3 mils, was obtained by the above process.

The use of an alkylolamine compound, such as monoethanolamine, in the manner described, markedly facilitates the entrance of the molten aluminum into the most minute crevices and corners of an intricately configurated part during the dipping process, apparently because of lowered surface tension effects. There thus results the formation of considerably improved aluminum coatings in these regions, which heretofore had been difficult to aluminize and which formed focal points for corrosion and rusting under operating conditions. Moreover, less time of immersion in the bath of such parts is necessary to provide effective aluminum coatings in the narrow spaces of the complex configurated portions. A further advantage of the present process is that it is unnecessary to add ingredients, such as sodium, to the bath to lower its surface tension as often done in the prior art, and the problems of handling, cost, and contamination attendant on the addition of such materials are consequently avoided.

The advantages of the invention are applicable to various intricately shaped or fabricated parts. Projectionwelded parts, seam-welded parts, and press-fitted parts treated with monoethanolamine have all resulted in complete aluminum fillets at the junction of the joined members, as Well as a continuous, void-free aluminum filling in the narrow spaces between the adjoining faces of the members. In the case of press-fitted parts, as, for example, a tube member pressed into an annular flange, this procedure provided an effective means for joining ferrous parts with an aluminum braze.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent to the United States is:

1. The method of coating with aluminum a metal body having a cleaned surface which comprises applying on the cleaned surface of the metal body a coating consisting essentially of an alkylolamine compound for preserving the cleaned surface, and thereafter applying molten aluminum to the thus treated surface.

2. The method of coating a ferrous metal body which comprises thoroughly cleaning the surface of the metal body, applying on the thus cleaned surface of the metal body a coating consisting essentially of an alkylolamine compound for preserving the cleaned surface, and thereafter applying molten aluminum to the thus treated surface.

3. The method of forming a continuous tightly adherent coating of aluminum on a ferrous base metal which comprises thoroughly cleaning the surface of the base metal, applying a coating consisting essentially of an alkylolamine compound thereon for preserving the cleaned surface, and immersing the thus treated surface of said base metal in a bath of molten metal consisting essentially of aluminum.

4. The method of forming a continuous adherent coating of aluminum on the surface of a ferrous metal article of relatively complex shape having narrow spaces, which comprises thoroughly cleaning the surface of the metal article, applying a coating consisting essentially of an alkylolamine solution upon the cleaned surface and drying the same for preserving the cleaned surface, immersing the thus treated surface of said ferrous article in a bath of molten aluminum, whereby the molten aluminum readily penetrates into the narrow spaces of the article, and withdrawing the article from the bath.

5. The method as defined in claim 4, wherein the alkylolamine is monoethanolamine.

6. The method of forming a continuous tightly adherent coating of aluminum on a ferrous base metal which comprises subjecting the ferrous base metal to a cleaning treatment including pickling the same in an acid solution, providing a coating consisting essentially of monoethanolamine on the thus cleaned surface of said ferrous base metal for preserving the cleaned surface, and dipping the thus treated ferrous base metal in a molten aluminum bath.

7. In a hot-dip coating process wherein a cleaned metal article is dipped in molten aluminum, the step of applying on the cleaned article a coating consisting essentially of an aqueous solution containing .ll0% by volume of monoethanolamine for preserving the cleaned surface before dipping the article in the molten aluminum.

References Cited in the tile of this patent UNITED STATES PATENTS 2,238,068 Miller Apr. 15, 1941 

1. THE METHOD OF COATING WITH ALUMINUM A METAL BODY HAVING A CLEANED SURFACE WHICH COMPRISES APPLYING ON THE CLEANED SURFACE OF THE METAL BODY A COATING CONSISTING ESSENTIALLY OF AN ALKYLOLAMINE COMPOUND FOR PRESERVING THE CLEANED SURFACE, AND THEREAFTER APPLYING MOLTEN ALUMINUM TO THE THUS TREATED SURFACE. 